Saturated core fault current limiters (FCLs) are known. Examples of superconducting fault current limiting devices include:                U.S. Pat. No. 7,193,825 to Darmann et al.        U.S. Pat. No. 6,809,910 to Yuan et al.        U.S. Pat. No. 7,193,825 to Boenig.        US Patent Application Publication Number 2002/0018327 to Walker et al.        
The fault current limiters described are for use with dry insulation type copper coil arrangements and, in practical terms, only suitable for DC saturated FCLs which employ air as the main insulation medium. That is, the main static insulation medium between the AC phase coils in a polyphase FCL and between the AC phase coils and the steel core, DC coil, cryostat, and main structure is provided by a suitable distance in air. This substantially limits the FCL to a “dry type” insulation technologies. Dry type technologies normally refers to those transformer construction techniques which employ electrically insulated copper coils but only normal static air and isolated solid insulation barrier materials as the balance of the insulation medium. In general, air forms the majority of the electrical insulation material between the high voltage side and the grounded components of the FCL. These grounded components include the steel frame work and the case.
The utilisation of dry type insulation limits the FCL to lower voltage ranges of AC line voltages of up to approximately 39 kV. Dry type transformers and reactors are only commercially available up to voltage levels of about 39 kV. As a result, the current demonstrated technology for DC saturated FCL's is not suitable for extension into high voltage versions. Dry type designs result in an inability to design a practically sized compact structure using air as an insulation medium when dealing with higher voltages.
One of the main emerging markets for FCL's is the medium to high voltage (33 kV to 166 kV) and extra-high voltage range (166 kV to 750 kV). When operating within these voltage ranges, the currently described art and literature descriptions of DC saturated FCL's are not practical. The main reason is due to static voltage design considerations—for example, the breakdown of the air insulation medium between the high voltage copper coils and the cryostat or steel core or DC coil. High voltage phase coils at medium to high voltages (greater than 39 kV) often need to be immersed in one of                An insulating gas (such as SF6, nitrogen, or the like).        A vacuum (better than 10−3 mbar).        A liquid such as a synthetic silicone oil, vegetable oil, or other commonly available insulating oils used in medium, high voltage, and extra-high voltage transformer and reactor technology.        
When a high voltage device is immersed in such an insulating medium, that medium is often referred to as the “bulk insulation medium” or the “dielectric”.
Typically, the dielectric will have a relative permittivity of the order of about 2 to 4, except for a vacuum which has a relative permittivity equal to 1. These so called dielectric insulation media have electrostatic breakdown strength properties which are far superior to that of atmospheric air if employed judiciously by limiting the maximum distance between solid insulation barriers and optimising the filled dielectric distance with respect to the breakdown properties of the particular liquid or gaseous dielectric.
The commonly available bulk insulating gases and liquids typically have a breakdown strength in the order of 10 to 20 kV/mm but are usually employed such that the average electric field stress does not exceed about 6 to 10 kV/mm. This safety margin to the breakdown stress value is required because even if the average electrostatic field stress is 6 to 10 kV/mm, the peak electrostatic field stress along any isostatic electric field line may be 2 to 3 times the average due to various electrostatic field enhancement effects.
In general, there are five main desirable requirements of a dielectric liquid or gas for high voltage bulk insulation requirements in housed plant such as transformers and reactors and fault current limiters:                The dielectric must show a very high resistivity.        The dielectric losses must be very low.        The liquid must be able to accommodate solid insulators without degrading that solid insulation (for example, turn to turn insulation on coil windings or epoxy).        The electrical breakdown strength must be high.        The medium must be able to remove thermal energy losses.        
Solid insulation techniques are not yet commonly available at medium to high voltages (that is, at operating voltages greater than 39 kV) for housed devices such as transformers, reactors and fault current limiters. The shortcoming of solid insulation techniques is the presence of the inevitable voids within the bulk of the solid insulation or between surfaces of dissimilar materials such as between coil insulation and other solid insulation materials. It is well known that voids in solid insulation with high voltages produce a high electric stress within the void due the field enhancement effect. This causes physical breakdown of the surrounding material due to partial discharges and can eventually lead to tracking and complete device failure.
It will be recognized that a DC saturated fault current limiter which employs a single or multiple DC coils for saturating the steel core, such as those disclosed in the aforementioned prior art, poses fundamental problems when the copper AC phase coils can no longer be of a “dry type” construction or when the main insultion medium of the complete device is air. A significant problem in such arrangements is the presence of the steel cryostat for cooling the DC HTS coil and the DC HTS coil itself. The cryostat and the coil and the steel cores are essentially at ground potential with respect to the AC phase coils.
As a side issue, but one which enhances the insulation requirements for all high voltage plant and equipment, it is that basic insulation design must also meet certain electrical engineering standards which test for tolerance to various types of over-voltages and lighting impulses over predetermined time periods. An example, in Australia, of such standards are as follows:                AS2374 Part3. Insulation levels and dielectric tests which includes the power frequency (PF) and lightning impulse (LI) tests of the complete transformer.        AS2374 Part 3.1. Insulation levels and dielectric tests—External clearances in air.        AS2374 Part 5. Ability to withstand short-circuit.        
These standards do not form an exhaustive list of the standards that high voltage electric equipment must meet. It is recognised that each country has their own standards which cover these same design areas and reference to an individual country's standard does not necessarily exclude any other country's standards. Ideally a device is constructed to meet multiple countries standards.
Adherence to these standards result in a BIL (Basic Insulation level) for the device or a “DIL” (Design Insulation Level) which is usually a multiple of the basic AC line voltage. For example, a 66 kV medium voltage transformer or other housed device such as a FCL may have a BIL of 220 kV. The requirement to meet this standard results in a static voltage design which is more strenuous to meet practically than from a consideration of the AC line voltage only. The applicable standards and this requirement has resulted from the fact that a practical electrical installation experiences temporary over voltages which plant and devices may experience within a complex network, for example lightning over voltages, and switching surges. Hence, all equipment on an electrical network has a BIL or DIL appropriate for the expected worst case transient voltages.
An initial consideration of the static design problem for high voltage DC saturated fault current limiters may result in the conclusion that the problem is easily solved by housing only the high voltage AC copper coils in a suitable electrical insulating gas or liquid. However, the problem with this technique is that the steel core must pass through the container which holds the gas or liquid. Designing this interface for long term service is difficult to solve mechanically. However, more importantly solving the interface problem electrostatically is much more complex and any solution can be prone to failure or prove uneconomical. The problem is that as a seal must be developed between the vessel containing the dielectric fluid and the high permeability core or, alternatively, a method of isolating the HTS cryostat from the fluid.
Another possibility is the use of solid high voltage barriers between phases and between phases and the steel core and cryostat or a layer of high voltage insulation around the copper phase coils and in intimate contact with the phase coils. However, this has a significant deleterious side effect. It is known that the static electric field in a combination of air and other materials with a higher relative permittivity is that this always results in an enhanced electric field in the material or fluid with the lower permittivity (that is air). For example, consider a conductive copper cylinder with a layer of normal insulation to represent the turn to turn insulation, according equation 1:
                              E          x                =                              U            m                                x            ⁢                          {                                                                    ln                    ⁡                                          [                                              R                        r                                            ]                                                                                                  ɛ                      2                                        /                                          ɛ                      1                                                                      +                                                      ln                    ⁡                                          [                                              d                        R                                            ]                                                        1                                            }                                                          Equation        ⁢                                  ⁢        1            where:                Um=AC phase voltage with respect to ground.        R=radius of a copper cylinder including outside insulation [mm].        r=radius of bare copper cylinder [mm].        d=distance from centre of cylinder to the nearest ground plane [mm].        ∈2=relative dielectric constant of the insulation covering the cylinder        ∈1=relative dielectric constant of the bulk insulation where the cylinder is immersed (which equals 1 for air).        x=distance from the centre of cylinder to a point outside the cylinder [mm].        Ex=Electrostatic field gradient at point x [kV/mm].        
The field enhancement effect is represented by the factor ∈2/∈1 and is of the order 2 to 4 for common everyday materials except for the case of employing a vacuum which has a relative permittivity equal to 1. By providing additional solid or other insulation material (of higher electric permittivity than air) there is an increase in the electrostatic stress in the bulk air insulation of the FCL. The better the quality of the high voltage insulation, the higher the field enhancement effect.
Hence, solid dielectric insulation barriers in an otherwise air insulated FCL are not a technically desirable option for high voltage FCL's at greater than 39 kV and indeed one does not see this technique being employed to make high voltage dry type transformers at greater than 39 kV for example. In fact, no techniques have been found highly suitable to date and that is why high voltage transformers above 39 kV are insulated with a dielectric liquid or gas.
The discussion above is the reason why housed high voltage electrical equipment is often completely immersed in electrically insulating dielectric fluid or gas. That is, the insulated copper coils and the steel core of transformers and reactors are housed within a container that is then completely filled with a dielectric medium which is a fluid. This substantially reduces the electrostatic voltage design problems detailed in the above discussion. The insulating medium (for example oil, vacuum, or SF6) fills all of the voids and bulk distances between the high voltage components and the components which are essentially at ground or neutral potential. In this case, solid insulation barriers may be incorporated into the bulk insulating dielectric and for many liquids such as oil, dividing the large distances with solid insulation improves the quality of the overall electrostatic insulation by increasing the breakdown field strength of the dielectric fluid. This is because the relative permittivity of the oil and solid insulation are very close to each other (so field enhancement effects are lessened compare to air) and the breakdown voltage of the bulk dielectric medium (expressed in kV/mm) improves for smaller distances between the insulation barriers.
A major problem with the full immersion technique is that it is not readily adaptable to a DC saturated FCL designs or other devices that incorporated a superconductor coil as the DC saturating element. This is because the superconducting coil and its cryostat or vacuum vessel are a component of the FCL which must also necessarily be immersed in the dielectric fluid.
The established body of literature clearly points to four main criteria for a marketable, feasible, and manufacturable FCL:                It must have a low insertion impedance so that it is invisible to the network when there are no faults and when providing peak power flow.        It must not produce more than 0.5% THD worth of harmonics (Total harmonic distortion) or as required by the end user.        It must provide a suitable clip of the fault current, between 20 to 80%.        The design must be augmentable to high AC voltages (greater than 6 kV) and high AC current (greater than 0.6 kA).        
The classic saturable core FCL designs detailed in the prior art suffer the major drawbacks of not being suitable for high voltage and high AC current designs. Both of these disadvantages originate from the lack of a coolant (other than air) and/or a liquid or gaseous dielectric.
Even if a liquid or gaseous dielectric is employed in the classic saturable FCL design, there is still required significant augmentation to allow access to the cryocooler, cryostat, and cryostat fittings. In addition, special seals to isolate the cryostat feed-throughs (electrical power, electrical signals) from the dielectric have to be made and tested.
In high AC current designs, the cross sectional area of copper required to conduct the required electrical current is much higher when considering only an air cooled design. It is not unusual for this cross section area to be up to five times higher. This can make the dimensions of the AC coil too large to be accommodated into the minimum core frame yoke size, requiring a larger yoke to maintain electrostatic clearance. This increases the footprint and mass of the classic air cooled/air insulated saturable FCL.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.